In high-axle load railways mainly transporting mineral ores, a load on an axle of a freight car is significantly higher than that of a passenger car, and the use environment of rails is also severe. Rails used in such an environment have been mainly composed of steel having a pearlitic structure from the viewpoint of significant concern of wear resistance. To enhance the efficiency of railway transport, progress has recently been made in increasing carrying capacity. Thus, there is a need for further improvement in wear resistance and rolling contact fatigue resistance. High-axle load railways are used to indicate railways in which trains and freight cars have a large carrying capacity (for example, a carrying capacity of about 150 ton or more per freight car).
In recent years, various studies have been conducted to further improve wear resistance. For example, in Japanese Unexamined Patent Application Publication Nos. 8-109439 and 8-144016, the C content is increased to more than 0.85% and 1.20% by mass or less. In Japanese Unexamined Patent Application Publication Nos. 8-246100 and 8-246101, the C content is increased to more than 0.85% to 1.20% by mass or less and a rail head is subjected to heat treatment. In this way, for example, a technique for improving wear resistance by increasing the C content to increase the cementite ratio has been used.
Meanwhile, rails placed in curved sections of high-axle load railways are subjected to rolling stress due to wheels and slip force due to centrifugal force, causing severe wear of rails and fatigue damage due to slippage. As described above, in the case where the C content is simply more than 0.85% and 1.20% by mass or less, a proeutectoid cementite structure is formed depending on heat treatment conditions, and the amount of a cementite layer in a brittle lamellar pearlitic structure is also increased; hence, rolling contact fatigue resistance is not improved. Japanese Unexamined Patent Application Publication No. 2002-69585, thus, discloses a technique for inhibiting the formation of proeutectoid cementite by addition of Al and Si to improve rolling contact fatigue resistance. The addition of Al, however, causes the formation of an oxide acting as a starting point of fatigue damage, for example. It is thus difficult to satisfy both wear resistance and rolling contact fatigue resistance of a steel rail having a pearlitic structure.
To improve the operating life of rails, in Japanese Unexamined Patent Application Publication No. 10-195601, a portion located from the surface of corners and of the top of the head of the rail to a depth of at least 20 mm have a hardness of 370 HV or more, thereby improving the operating life of the rail. In Japanese Unexamined Patent Application Publication No. 2003-293086, by controlling a pearlite block, a portion located from the surface of corners and of the top of the head of the rail to a depth of at least 20 mm have a hardness of 300 HV to 500 HV, thereby improving the operating life of the rail.
The use environment of pearlitic rails, however, has been increasingly severe. To improve the operating life of pearlitic rails, there have been a challenge for higher hardness and the expansion of the range of hardening depth.